The present invention relates to a semiconductor device, and particularly relates to a structure of a light detection element in an image sensor in which a light detection element and a thin film transistor are formed on the same substrate, and in which the thin film transistor is protected to thereby improve the yield.
Conventionally, a contact-type image sensor is used for projecting and converting picture information of an original document into electric signals having one to one correspondence. There has been proposed a TFT-driving image sensor in which a projected picture is divided into a number of picture elements, and charges produced in respective light detection elements corresponding to the picture elements are accumulated temporarily in wiring capacities of respective wirings for every specific block by switching elements constituted by thin film transistors (TFT), and read out as electric signals by a driving IC sequentially in time series at a rate in a range of from several hundred KHz to several MHz. In this TFT image sensor, by a matrix operation performed by TFTs, it is possible to read light detection elements in a plurality of blocks by a single driving IC, so that it is possible to reduce the number of driving ICs for driving an image sensor.
A TFT-driving image sensor is, for example, as shown in an equivalent circuit thereof in FIG. 3, constituted by a light detection element array 101 in which a plurality of light detection elements P(k, n) are arraged in a line having almost the same length as the width of an original document, a charge transfer portion 102 constituted by thin film transistors T(k, n) which one to one correspond to the respective light detection elements P(k, n), and a matrix-shaped multilayer wiring 103.
The light detection element array 101 is divided into k blocks of light detection element groups, and n light detection elements P(k, n) constituting each group of light detection elements can be expressed equivalently by photodiodes and parasitic capacities. The respective light detection elements P(k, n) are connected with drain electrodes of the corresponding thin film transistors T(k, n). The respective source electrodes of the thin film transistors T(k, n) are connected with the corresponding common signal lines 104 (n lines) for every light detection element group through the multilayer wiring 103 connected in a matrix, and the common signal lines 104 are connected with a driving IC 105. The gate electrodes of the respective thin film transistors T(k, n) are connected with a gate pulse generating circuit 106 so that the thin film transistors T(k, n) are made conductive for every block.
Photoelectric charges produced in the respective light detection elements P(k,n) are accumulated in parasitic capacities of the light detection elements P(k, n) and overlap capacities between the drain electrodes and gate electrodes of the thin film transistors T(k, n) for a constant time, and thereafter are transferred to and accumulated in wiring capacities CL of the multilayer wiring 103 sequentially for every block in such a manner that the thin film transistors T(k, n) are used as switches for transferring charges. That is, a gate pulse .phi.G1 transferred from the gate pulse generating circuit 106 through a gate signal line G1 makes thin film transistors T(1, 1) to T(1, n) in the first block turn on, so that charges produced in the respective light detection elements P(k, n) in the first block are transferred to and accumulated in the respective wiring capacities CL. Then the charges accumulated in the respective wiring capacities CL change the electric potentials of the respective common signal lines 104, and analog switches SWn in the driving IC 105 are turned on sequentially to extract these potential values through an output line 107 in time series. Then gate pulses .phi. G2 to .phi.Gk turn on the thin film transistors T(2, 1) to T(2, n) of the second block through the thin film transistors T(k, 1) to T(k, n) of the k-th block respectively, so that charges on the light detection elements are transferred for every block and read sequentially to obtain picture signals of one line of the main scanning direction of an original document. The above-mentioned operation is repeated while the original document is shifted by an original document feeding means (not-shown) such as a roller, so that picture signals of the whole of the original document can be obtained (reference be made to Japanese Unexamined Patent Publication No. Sho-63-9358).
A light detection element P in the above-mentioned image sensor and a thin film transistor T provided for every light detection element P for transferring charges produced in the light detection element P are formed on the same glass substrate 1 as shown in FIGS. 2A to 2D. A production process of the light detection element P and the thin film transistor T will be described with reference to FIGS. 2A to 2D.
First, a chromium layer (Cr) is formed on a glass substrate 1 and patterned to thereby form a gate electrode 2.
Next, a silicon nitride film (SiNx) to be formed into a gate insulation layer 3, an amorphous silicon hydride (a-Si:H) film 4' to be formed into a semiconductor active layer 4, and a silicon nitride (SiNx) film are successively formed, and then the silicon nitride (SiNx) film is patterned to thereby form an upper insulation layer 5 on the gate electrode 2.
Succeedingly, an n.sup.+ amorphous silicon hydride (n.sup.+ a-Si:H) film 6', a metal film 7' to be formed into a lower electrode of the light detection element P and a barrier metal layer of the thin film transistor, an amorphous silicon hydride (a-Si:H) film 8', and an indium-tin oxide (ITO) film 9' are formed continuously (FIG. 2A).
After a resist (not shown) is formed on the indium-tin oxide film 9', the pattern of a transparent electrode 9 of the light detection element P is formed by an etching process (FIG. 2B).
Next, the amorphous silicon hydride film 8' is patterned by an etching process so as to form a photoconductive layer 8 of the light detection element P (FIG. 2C).
Next, the metal film 7' is patterned by a photo-lithograph process so as to form a lower electrode 7a of the light detection element P and barrier metal layers 7b and 7c of the thin film transistor T. Succeedingly, the n.sup.+ amorphous silicon hydride film 6, is patterned with the same mask so as to form ohmic contact layers 6b and 6c of the thin film transistor T, and further the amorphous silicon hydride (a-Si:H) film 4, is patterned so as to form the semiconductor active layer 4 of the thin film transistor T (FIG. 2D).
In the above-mentioned production process, the metal film 7' serves also as an etching stopper at the time of etching the amorphous silicon hydride film 8' to form the photoconductive layer 8, as shown in FIG. 2C. Therefore the metal film 7' is formed of a material such as, for example, chromium (Cr) or titanium (Ti), which cannot be etched at the time of etching the amorphous silicon hydride film 8'.
In the case where chromium (Cr) is used as the metal film 7', it becomes a good etching stopper at the time of etching the amorphous silicon hydride film 8', but it is apt to be melted by electrolytic corrosion, so that there has been a problem that the reliability of the light detection element P or the thin film transistor T is deteriorated.
On the other hand, in the case where titanium (Ti) is used as the metal film 7', it is apt to produce a reaction in the interface between the amorphous silicon hydride film 8, and the metal film 7' of titanium (Ti) to thereby form silicide, and this silicide is etched under the etching conditions of the amorphous silicon hydride film 8', so that there has been a problem that the yield of the thin film transistor T formed under the metal film 7' is deteriorated.